1. Field of the Invention
The present invention relates to an optical web inspection apparatus and in particular to an optical web inspection apparatus able to monitor variations in thickness of a moving film web.
2. Description of the Prior Art
It is known in the art to utilize the interferometric effect created by the first and second surfaces of a traveling film web to generate an optical fringe pattern and to utilize the same in the determination of a predetermined physical property, typically the thickness in absolute terms, of the traveling web. Exemplary of such apparatus is that described in U.S. Pat. No. 3,319,515 (Flournoy) assigned to the assignee of the present invention. However, the system described in this patent does not provide a mechanism whereby small variations (in the range of less than one percent) in thickness of the traveling web can be accurately gauged while on-line, i.e., without removing the web from the manufacturing line.
Control systems which utilize interferometric phenomena are known. Exemplary of such systems are those shown in U.S. Pat. Nos. 3,494,698 (Neumann) and 3,963,063 (Krogstag et al.). These systems use monochromatic light
These systems use monochromatic light sources and are, thus, not compatible for an inspection system in which on-line monitoring of a web may be effected because the film cannot itself be used as the interferometer.
Accordingly, in view of the foregoing it is believed advantageous to provide an apparatus for monitoring, on-line, thickness variations in a traveling web.